1. Field of the Invention
The present invention relates to a method of forming a deposition film such as a photoconductive film, a semiconductor film or an insulating film on a predetermined substrate utilizing light as an excitation energy, more particularly to a method of forming a deposition film wherein a source gas is excited or decomposed by applying excitation energy such as one or both of light and heat so as to form a deposition film containing silicon atoms on a predetermined substrate.
2. Description of the Prior Art
As a conventional amorphous silicon (to be referred to as "a-Si" hereinafter) deposition film formation method, the glow discharge deposition method or thermal energy deposition method utilizing SiH.sub.4 or Si.sub.2 H.sub.6 as a raw material for a source gas is known. In these methods, SiH.sub.4 or Si.sub.2 H.sub.6 as a source gas is decomposed by electrical or thermal energy (excitation energy) so as to form an a-Si deposition film on a substrate. The deposition film formed in this manner is used for various purposes such as a photoconductive film, a semiconductor film or an insulating film.
However, in the glow discharge deposition method wherein a deposition film is formed with a high-output discharge, a uniform discharge distribution cannot always be obtained and stable condition control with good reproducibility is difficult to attain. In addition, the adverse influence of high-output discharge on the film being formed is considerable. Thus, it is difficult to impact uniform electrical and optical characteristics and stable quality to the formed film. The formed film therefore tends to have an irregular surface or microdefects therein. It is particularly difficult to form a deposition film having a large area or thickness and uniform electrical and optical characteristics.
In the thermal energy deposition method, the raw material is normally subjected to a high temperature of 400.degree. C. or higher. For this reason, the substrate material which can be used is limited. In addition, the probability that effective bonded hydrogen atoms in desired a-Si are dissociated is increased, again resulting in poor characteristics of the film.
As a method free from the problems with these methods, the photo-energy deposition method (to be referred to as the "photo-CVD" method hereinafter) of depositing a-Si using SiH.sub.4 or Si.sub.2 H.sub.6 as a raw material is receiving a lot of attention recently.
The photo-CVD method uses light in place of the glow discharge or heat as the excitation energy unlike in the above methods. For this reason, an a-Si deposition film can be formed at a low energy level and it is easy that a source gas can be uniformly irradiated with light energy. As compared to the above-mentioned deposition method, a high- and uniform-quality film can be formed with a lower energy consumption than with the above methods. Control of film formation conditions is also easy to achieve stable reproducibility. In addition, a substrate need not be heated to a high temperature and a substrate material can be selected from a wider range of materials.
However, this photo-CVD method using SiH.sub.4 or Si.sub.2 H.sub.6 as a raw material has only a limited decomposition efficiency of the source gas and hence a limited deposition rate, thus disabling mass-production.